1. Field of the Invention
The present invention relates to the field of digital video technology and, more particularly, to a method and apparatus adapted to remove color artifacts at boundaries between subject and non-subject image portion in region-based video coding.
2. Background
Video sequences comprised of multiple video frames are processed by digital video coding systems prior to being transmitted as a bitstream over a channel of limited bandwidth. The bitstream includes video sequence data comprising pixel values that define the manner in which each video frame is generated onto a display. The bandwidth of the channel restricts the transmission rate of the bitstream (and video sequence date). A digital video coding system is employed to process the video sequence data in order to reduce the amount of bits or the amount of data included in the bitstream. Generally, however, the process employed by the digital video coding system, e.g., video sequence data reduction, has a negative impact on the quality of the transmitted video sequence.
Image segmentation techniques have been employed by some prior art systems to lessen the negative impact on the quality of selected regions and the transmitted video sequence. Image segmentation involves separating or distinguishing regions depicted in the video sequence in the form of segmentation data. Segmentation data includes data for indicating the region to which specific pixels in the video sequence belong. Prior art digital video coding systems utilize image segmentation techniques to reduce portions of the video sequence data according to specific regions. In other words, the quality of each region in the video sequence can be individually specified.
Referring to FIG. 1, there is shown a representation of a typical prior art digital video coding system. As shown, the digital video coding system determines the segmentation data for the regions depicted in a video sequence using an image segmenter 102. An encoder 104 encodes the video sequence along with the segmentation data provided by the image segmenter 102. The manner in which the video sequence data is to be encoded can be specified using the segmentation data such that selected regions have a higher degree of quality than other regions. Subsequently, the encoded video sequence data and segmentation data are transmitted to a decoder 106 proximate a display device, for example, where the encoded video sequence data and segmentation data are decoded and the video sequence data is recovered.
For some prior art systems, the price for improving the quality of selected regions in the video sequence is the transmission of additional bits. As shown in FIG. 1, the encoded segmentation data is included in the bitstream along with the encoded video sequence data because the encoded segmentation data is necessary to recover the video sequence. In other words, the segmentation data is normally needed to determine the manner in which the video sequence data was encoded such that the video sequence data can be re-created or recovered. In other prior art systems, the encoded segmentation data may not be necessary to recover the video sequence. However, these prior art systems do not provide content based scalability.
All prior art systems utilizing image segmentation techniques to improve the quality of selected regions in a video sequence require special encoders. Industry standard encoders are operative to encode the video sequence data using the well-known M.P.E.G., H.261 or H.263 (under development) compression algorithms which encode the video sequence data using some combination of discrete cosine transformation and motion compensation. The region to which a particular pixel belongs is irrelevant to these compression algorithms. The exemplary encoder 104 of the prior art system thus employs a different compression algorithm for encoding the video sequence according to region. This requires the encoder 104 to perform an extra process before the video sequence data can be encoded. Specifically, the encoder 104 must first determine which region each pixel value belongs to before the video sequence data can be encoded. These types of encoders have not been widely accepted by the industry. Accordingly, there exists a need for a digital video coding system that employs an industry standard encoder to lessen the negative impact on the quality of the transmitted video sequence while still transmitting fewer bits.